Liquid crystal reflective displays

ABSTRACT

In a liquid crystal reflecting device comprising a cell ( 1 ) formed by two cell walls ( 3, 4 ) spaced apart by spacers ( 5 ) to contain a layer ( 2 ) of a liquid crystal material, electrode structures ( 6, 7 ) on the inner faces of the walls, and a surface alignment on one or both walls to align the liquid crystal material, a diffuse specular reflector is provided by a mirror base layer ( 21 ) formed by at least two different materials that are at least partly immiscible relative to one another forming a rough surface onto which a reflective layer ( 22 ) is coated. The mirror base layer ( 21 ) is formed by at least two different polymer and/or monomer materials that are at least partly immiscible relative to one another. The reflector layer ( 22 ) may be a single sheet, formed into a plurality of separate reflector elements, or patterned into electrodes.

[0001] This invention relates to liquid crystal reflective displays.

[0002] Typically liquid crystal displays comprise a cell formed by twoglass walls spaced apart to contain a thin layer of a liquid crystalmaterial. Electrode structures are formed on the surface of the walls,for example the electrodes may be in the form of strips electrodesforming row and columns giving a matrix of addressable pixels at theintersections of each row and column. The display may be addressed a rowat a time in a known multiplex manner.

[0003] The liquid crystal material may be nematic or long pitchcholesteric to form conventional 90° twisted nematic or super twistednematic devices (270° twist as in U.S. Pat. No. 4,596,446), cholestericto form phase change devices, and smectic material to form ferroelectricdevices. Typical ferroelectric devices include the surface stabiliseddevices (SSFLCD) giving bistable devices. Such ferroelectric displaysare described for example in:—N A Clark and S T Lagerwal, AppliedPhysics Letters Vol 36, No 11 pp 889-901, June, 1980; GB-2,166,256-A;U.S. Pat. No. 4,367,924; U.S. Pat. No. 4,563,059; patent GB-2,209,610; RB Meyer et al,. J Phys Lett 36, L69, 1975.

[0004] Another bistable device is the zenithal bistable nematic device(ZBD™) as described in WO-97/14990, PCT/GB96/02463, GB98/02806.1, andEP96932739.4. This uses a special grating surface alignment on a cellwall to allow nematic or long pitch cholesteric material to form twostable states with two different tilted alignments.

[0005] Some devices are transmissive device operating with a backlight.Others are reflective devices operating with ambient light and requiringa rear mirror. Still others operate in both transmissive and reflectivemodes using a semireflective mirror and a backlight.

[0006] The present invention concerns devices using a rear mirror,either a totally or a partly reflecting mirror. Present mirrors may beformed by evaporating or spraying a reflective layer of e.g. aluminiumon the rear outer wall of the cell. To form a diffuse specularreflector, it is standard to abrade the rear wall to roughen the surfaceprior to deposition of a reflective layer. Such an abrading step is notcompatible with conventional clean room processing techniques.

[0007] According to the present invention, this problem is overcome bydeposition of a polymer layer, which forms a rough surface, then coatingthis rough surface with a reflective layer.

[0008] According to this invention a liquid crystal device comprises acell formed by two cell walls spaced apart by spacers to contain a layerof a liquid crystal material, electrode structures on the inner faces ofthe walls, and a surface alignment on one or both walls to align theliquid crystal material,

[0009] characterised by:

[0010] a mirror base layer formed by at least two different materialsthat are at least partly immiscible relative to one another forming arough surface onto which a reflective layer is formed to provide adiffuse specular reflector.

[0011] The layer may be a mixture of two or more monomers and orpolymers of thickness between 5 and 200 nano metres, typically 10 to 50,e.g. about 30 nm. Varying the relative proportions of the mixture and/orthe materials used varies the roughness of the resulting surface. Therough surface has a large number of minute protrusions and/or holes.When used inside a cell, the resulting rough surface is preferablyplanarised to give a uniform liquid crystal layer thickness and henceuniform appearance at each pixel. For some devices, the planarisationlayer may be omitted so that partial switching and/or greyscale may beobtained.

[0012] The reflector may be on the rear wall inside or outside the cell.The reflector may be formed into electrodes or may be independent of theelectrodes; in the latter case, the reflector may be a single sheet orpatterned into a plurality of separate reflector elements in registerwith pixels defined by electrode intersections and may include lightabsorbent material between the reflective elements.

[0013] The liquid crystal material may be nematic, long pitchcholesteric, cholesteric, or smectic with or without chirality.

[0014] Use of two different polymers that are immiscible is described inGB-2,33,076-A to form an uneven surface with differing alignmentproperties for providing partial switching and greyscale in an SSFLCD.The polymer surface is deposited over electrodes on a cell wall. Afterdeposition, the polymer surface is rubbed to give alignment.

[0015] The reflective layer may be a layer of any reflecting material,e.g. aluminium, silver, gold etc. between 1 and 100 mn thin, typicallyabout 70 to 80 nm, depending upon whether or not a fully or partlyreflecting mirror is required.

[0016] The invention will now be described by way of example only withreference to the accompanying drawings in which:

[0017]FIG. 1 is a plan view of a conventional twisted nematic matrixmultiplexed addressed liquid crystal display;

[0018]FIG. 2 is the cross section of the display of FIG. 1;

[0019]FIG. 3 is a cross section of one wall of a display according tothis invention in which a reflector also forms a set of electrodes;

[0020]FIG. 4 is a cross section of another wall according to thisinvention;

[0021]FIG. 5 is a cross section of another wall according to thisinvention;

[0022] The display in FIGS. 1, 2 comprises a liquid crystal cell 1formed by a layer 2 of nematic or long pitch cholesteric liquid crystalmaterial contained between glass walls 3, 4. A spacer ring 5 maintainsthe walls typically 3-12 μm apart. Additionally numerous beads of thesame dimensions may be dispersed within the liquid crystal to maintainan accurate wall spacing. Strip like row electrodes 6 e.g. of SnO₂ orITO (indium Tin Oxide) are formed on one wall 3 and similar columnelectrodes 7 are formed on the other wall 4 in a known manner. Withm-row and n-column electrodes this forms an m×n matrix of addressableelements or pixels. Each pixel is formed by the intersection of a rowand column electrode. The electrodes are covered with a thin layer 17 ofpolymer e.g. polyimide, which has been rubbed to give alignment to theliquid crystal material 2. For twisted nematic devices the alignmentdirections on the two walls are approximately orthogonal.

[0023] A row driver 8 supplies voltage to each row electrode 6.Similarly a column driver 9 supplies voltages to each column electrode7. Control of applied voltages is from a control logic 10, whichreceives power from a voltage source 11 and timing from a clock 12.

[0024] Either side of the cell 1 are polarisers 13, 14 arranged withtheir polarisation axis substantially crossed with respect to oneanother and substantially parallel to the alignment directions on theadjacent wall 3, 4. Additionally one or more compensation layers 19, ofe.g. stretched polymer are arranged between a cell wall under apolariser, usually at the front of the cell.

[0025] A partly reflecting mirror 16 may be arranged behind the cell 1together with a light source 15. These allow the display to be seen inreflection and lit from behind in dull ambient lighting.

[0026] Bistable ferroelectric liquid crystal devices are similar inconstruction, but have a different thickness of a smectic liquid crystalmaterial, e.g. 1-5 μm thick, and a different angle between the surfaceinduced alignment directions. Polarisers are also angled differently,e.g. about 45°.

[0027]FIG. 3 shows a rear cell wall 20 of the present invention in whicha mirror is inside the cell. The wall 20 has formed thereon a layer 21of mixed polymers and a reflecting layer 22 of aluminium, which ispatterned into column electrodes 7. A barrier layer 23 of insulatingfilm of the polymer AT902 is formed on the reflecting layer 22 and isitself covered with a rubbed layer of polyimide to form an aligninglayer 24.

[0028] In one example, the layer 21 was formed by spin coating 1 partJSR Jals 212 (obtainable from JSR of Japan) alignment polymer in 1 partbutyrolactone solvent, and ⅔ part Hitachi LQT 120 (obtainable fromHitachi of Japan) alignment polymer in 1 ⅓ part NMP (N-methylpyrrolidinone) solvent onto the glass wall. Typically the layer is spincoated at 2500 rpm for 10 seconds, the baked at 100° C. for 30 minutesto evaporate the solvents followed by 200° C. for a further 60 minutesto polymerise the layer. These components JALS 212 and LQT 120 areimmiscible. The resultant layer has a relatively rough surface,typically having protrusions and or recesses of about 500-700 nm inwidth and 60 nm height/depth within larger feature sizes of about 1.5-3μm. Varying the proportions of Jals 212 and LQT between 1:4 and 4:1(and/or solvents) varies the size of the surface features and hence thedegree of scattering. The same solvent may be used for both polymers;e.g. NNP can be used for both.

[0029] The reflecting layer 22 may be a layer of 20 to 100 run or morethick alumimum. Alternatively the layer may be 20 to 100 nm silver. Thelayer 22 may be deposited as a complete layer (e.g. by evaporation,spraying or thick film deposition) then patterned using standardphotoresist processing into strip like column electrodes, typically 100μm wide spaced 20 μm apart. Alternatively, the electrodes may bedeposited through a mask to the required dimensions.

[0030]FIG. 4 is similar to FIG. 3 except that the reflective layer isnot formed into electrodes but is a single sheet or patterned into pixelshaped elements in register with pixels formed at intersections of rowand column electrodes 6, 7. A barrier layer 23 is again formed on thereflector 22 and row electrodes 6 formed on this barrier layer 23. Afurther barrier layer 25 is formed on the electrodes and covered with analignment layer of polyimide, which is rubbed to form an alignment layer24.

[0031]FIG. 5 shows a rear cell wall with a reflector formed on theoutside of the cell. The rear wall 20 carries a mixed layer 21 ofimmiscible polymers as in FIG. 3. The layer 21 is coated with areflecting layer 22 of aluminium.

[0032] In a further example the two alignment materials, JALS212 andLQT120, were mixed in the proportions 1:1 by weight prior to theresultant mixture being deposited on the substrate structure by spincoating. The alignment layer was otherwise produced in the same manneras in the preceding example. Recessed areas of one alignment materialwere produced within raised areas of the other alignment material, theraised areas typically having a width of 100-300 nm and a length of400-1,200 nm, and the recessed areas typically having a width of 50-200nm and a length of 400-1,200 nm. Furthermore the height differencebetween the recessed areas and the raised areas was about 20 nm.

[0033] Other polymers that phase segregate (separate out on drying) maybe used.

1. A liquid crystal device comprising a cell (1) formed by two cellwalls (3, 4) spaced apart by spacers (5) to contain a layer (2) of aliquid crystal material, electrode structures (6, 7) on the inner facesof the walls, and a surface alignment on one or both walls to align theliquid crystal material, characterised by: a mirror base layer (21)formed by at least two different materials that are at least partlyimmiscible relative to one another forming a rough surface onto which areflective layer (22) is formed to provide a diffuse specular reflector.2. The device of claim 1 wherein the mirror base layer (21) is a mixtureof two or more monomers and or polymers of thickness between 5 and 200nanometres.
 3. The device of claim 1 wherein the reflector (21, 22) ison the rear wall inside the cell.
 4. The device of claim 1 wherein thereflector (21, 22) is on the rear wall outside the cell.
 5. The deviceof claim 1 wherein the reflective layer (22) is a single sheet layer. 6.The device of claim 1 wherein the reflective layer (22) is formed into aplurality of electrodes (6).
 7. The device of claim 1 wherein thereflective layer (22) is formed into a plurality of individualreflectors in register with at least part of the electrode definedpixels.
 8. The device of claim 1 wherein the thickness of the reflectivelayer (22) is arranged to provide a partial reflector.